A diverter ball is a ball that is dropped or pumped through wellbore tubulars in a process known as diversion, and used during acidizing and fracturing operations. One common method of fracturing wells is to have a multitude of perforations open in the well that are exposed to the fracturing pressure being pumped into the well bore. The formation will begin to fracture behind a few of the perforations and the majority of the fracturing fluid will flow through these few perforations. Typically, initiating fractures requires more pressure then continuing an existing fracture so diverter balls are used to divert flow to other perforations. A percentage of diverter balls compared to the total number of perforations are dropped in a well (as an example, 10 balls are dropped at a time in a well with 100 perforations) and the balls theoretically will plug off the perforations which are taking the majority of flow because the flow is carrying the balls. Once the diverter balls are plugging the flowing perforations, the well bore pressure will increase until new fractures are started behind other perforations. More diverter balls are dropped until the majority of all of the perforated intervals are fractured. In other well operations, balls are used that function to open and close downhole valves in different fracturing zones, and may serve as temporary plugs to zones below the ball. In this sense the term balls includes bars, plugs, darts, and other shaped members, and are more generally referred to herein as well operating elements.
Diverter balls and fracturing elements are typically flowed to the surface or dropped to the bottom of the wellbore when their use is completed. If they are not degradable in the wellbore environment, there is a disincentive to their use. Dissolvable diverter balls are known in diversion operations, however, their rate or location of dissolution is not controlled. In a diversion process, a ball, dart, or other non-dissolvable fracturing element may land on a seat and be positioned effectively to divert fracturing fluid, acids, and/or proppants outwardly through crossover ports for flow through the annulus into the formation adjacent a perforated casing section. Multiple small dissolvable diverter balls are then flowed in to the tubing, typically with the fracturing fluid, and seat on perforations in the casing to divert fracturing fluids, acids, and proppants from a high flow area of perforations to a low flow area so that the low flow area receives the treatment fluid. Eventually the dissolvable material degrades, whether from mechanical action, contact with a fluid, heat, or combination thereof, or before dissolving they are pumped to the surface with well fluids, or dropped to the bottom of the wellbore.
It would be desirable in many well operations to have well operating elements that controllably dissolve either in rate of dissolution, location of the element, or both (or include a portion that controllably dissolves) in the wellbore environment, without having to resort to highly acid conditions, high temperatures, and mechanical milling. Elastomeric drop balls are known, as well as balls composed of epoxy resin cores with suitable light- and heavy-weight fillers to achieve the desired specific gravity expected in a well fluid. Filled epoxy resin balls may be coated or overmolded with nitrile or fluorocarbon rubber. These materials are purposely made non-degradable in a wellbore environment and their removal requires milling, extreme acidity, high temperatures, or some combination thereof, all of which involve considerable expense and safety issues.
None of the known drop balls, diverter balls, and the like have the ability to dissolve in a controlled fashion, and there is a need in the art for such an item, particularly an item that has both dissolvable components and non-dissolvable components, the latter retrievable from the wellbore, or degradable under user defined conditions.